1. Field of the Invention
This invention relates generally to a heat exchanger circuit, and, in particular, to a bypass valve having a pressure bypass function that is insensitive to temperature changes of fluid in the circuit.
2. Description of the Prior Art
At low temperatures and while the engine of a motor vehicle is warming-up, transmission fluid is highly viscous, resulting in nearly no flow through an oil the cooler. High viscosity and low flow rate can compromise transmission durability because cooler return oil is usually routed to the transmission lubrication circuit.
A variety of thermal bypass valves have been designed to allow oil to bypass the cooler and return to the transmission to maintain desired flow rates to the lubrication circuit. Many of these valves use a thermal motor/actuator to actuate the valve to provide this function. When the transmission is hot, the thermal bypass valve closes. If the cooler is hot, the cooler and lubrication systems will function as intended. If, however, ambient temperature is very cold, oil in the cooler remains cold, and oil flow will is low.
A variety of thermal bypass valves have been developed to allow a pressure bypass function, wherein the valve opens to bypass the cooler when the oil temperature is high and the pressure drop across the cooler exceeds a designed threshold. However, an inherent problem exists in many of these thermal bypass valves with pressure bypass functions. Most will provide the desired function with features that involve a spring load balanced against a piston, washer or valve having a piston shape and piston function with a pressure differential across it. Many of these valves have a spring load that is grounded to the thermal motor/actuator, which continues to move as the oil temperature increases. This grounded relation causes the spring load to be a function of temperature and the pressure relief temperature is also a function of temperature.
Unfortunately, when the oil is very hot and the ambient air conditions are such that the cooler is still frozen and not flowing oil, these shortcomings cause the pressure difference to be so high that the pressure bypass function will not be available, and the transmission is at risk due to a lack of lubricating oil.